1. Field of the Invention
The invention relates generally to the field of seismic exploration of subsurface rock formations. More specifically, the invention relates to high resolution, shallow depth seismic investigation to identify potential subsurface hazards.
2. Background Art
Explorationists are continually adopting new and improved techniques for seismic imaging of subsurface rock formations. In addition to applying traditional seismic processing techniques to novel acquisition geometries, the techniques of arraying and beam forming can be used to enhance seismic resolution for a variety of deep earth and shallow geotechnical investigations or applications. U.S. Patent Application publication No. 2009/0122645 filed by Guigné and Pace, for example, describes a star array for beamforming and steering at depth to identify non-specular features. Such a method leads to the potential for higher resolution structural imaging of deep earth reservoirs and surrounding geology (e.g. faults, stratigraphic inhomogeneities) and for the identification of near surface geohazards (e.g. shallow gas, hydrates, sediment liquefaction). The most important applications from a perspective of reservoir management are the capture of high resolution localization of seismic attributes, which could be used for characterizing complex stratigraphies to a specific volume that is much smaller than is possible with conventional three dimensional (3D) seismic methods.
In the case of near surface geotechnical site investigations in marine environments, the risk of unforeseen sub-seabed conditions is of particular concern to offshore installations as failure to identify sub-seabed hazards can lead to environmental disasters, significant cost overruns and adverse safety impacts. To mitigate such risks, geophysical and geotechnical site investigation procedures are executed before operations on a site begin. The investigations follow industry accepted standards that rely on interpretations of geological features based on interpolations between direct seismic soundings. Consequently, such investigations do not deliver results with resolution that engineering exactness may require. Nevertheless, industry accepted procedures for site evaluation are widely recognized as “best practices.” As offshore engineering and exploratory drilling projects become more numerous, and are conducted in ever greater water depths, there will be demand for acquiring more reliable, more detailed information concerning subsurface structure and stratigraphy. It is recognized that shortcomings exist in the site evaluation techniques that are currently being used. Therefore a reevaluation of what constitutes appropriate localized high resolution imaging of discrete targets is underway, be it for deep earth seismic mapping of scattering features or for near surface foundation related studies.
Geophysical and geotechnical site investigation methods recognized as industry standards use continuous seismic surveys, and borehole or well downhole imaging if available. Advantages of continuous seismic profiling include its ability to map the continuity of coherent sedimentary layers. In conventional seismic surveying, the resolution is determined by a combination of the dominant frequency propagated to and from reflectors (e.g., a reservoir) and by the algorithm(s) used to convert the time based data volume to a true depth volume. The vertical resolution is mostly controlled by the source frequency while the lateral resolution is much more controlled by the inversion (migration) algorithm. A particular problem emerges when the sedimentary or sub bedrock character becomes discontinuous because of discrete scattering bodies instead of continuous well defined coherent layered boundaries.
There continues to be a need for improved techniques for sub-bottom evaluation of marine operating sites to identify structural and stratigraphic features in detail.